1. Field of the Invention
The present invention relates to a molding method for obtaining a heat resistant container by putting a preform through a primary blow molding manufacturing step, a heat shrinking manufacturing step and a further secondary blow molding manufacturing step, whereby the lower region of this container retains the desired heat resistant characteristics.
2. Description of the Related Art
In, for example, U.S. Pat. No. 4,836,971, the following kind of technology for producing a heat resistant container is disclosed. An injection molded heated preform is primary blow molded into a primary blow molded article, which is longer than the final molded article. This is then reduced by heat shrinking to a length and width which is smaller than the final molded container. This smaller container is then secondary blow molded into a heat resistant container.
However, the aforementioned way of molding a bottle does not take into account the fact that the lower region needs to have both a strong resistance to being dropped and be heat resistant.
The present inventors have found the reason why such a bottle is weak in a drop impact is that wall thickness of a region of a bottle referred to as a heel is formed relatively thin.
This designated heel region ranges from the base formed from the outside edge of the bottom to the raised region. This heel region is the last region to arrive at the inner cavity surface of the secondary blow molding cavity when the heat shrunk molded article is secondary blow molded. That is to say that the sidewall part of the heel region which is just up from the heel region will make contact with the cavity wall before the heel does. Also, more particularly, when the raised shape of the bottom is concave, as with a champagne bottle, the bottom inner walls of this heel part will also make contact with the cavity wall before the heel part does. Also, the resin material which comes into contact with the cavity wall becomes more difficult to stretch afterwards. As a result of this, when the region corrosponding to the heel is then blow molded, this heel region will be thinner than the corrosponding wall regions.
If a load is then applied, for example, during a drop test, the heel could change shape or could collapse completely. Any change in the heel shape would be markedly detrimental to the bottles heat resistance and self-standing ability. Also, if the heel of the bottle was thin, this problem could be caused simply by applying pressure by hand.
The problem of the lower part of the bottle having a low resistance to heat occurs because of the comparatively wide scope of the low stretched region, but this will be cleared up by the present invention.
The low stretched region is formed during the primary blow molding process and already in the bottom portion of the primary molded article. If this primary blow molding article is put through a secondary blow molding manufacturing step after it has been heat shrunk, the low stretched region formed in the primary blow molded article will be more easily stretched than other regions. As a result of this the low stretched region will spread into a comparatively large area of the lower region of the secondary blow molded article. However, the stretch ratio during the secondary blow molding process is substantially lower than that during the primary blow molding process. This means that even if this low stretched region is stretched during the secondary blow molding process, its orientation will still be low when compared to other regions so its resistance to heat will be poor.